A hierarchal and tightly controlled organization of various cell types is the hallmark of normal tissues and organs, and the hypothesis underlying this proposal is that pre-defining the specific location and resultant cell- cell interactions of individual cells within a 3D tissue construct will allow one to create highly functional tissues in which the role of cell-cell interactions on cell phenotype can be precisely delineated. This concept will be explored by developing a 3D model of human hematopoiesis, in which osteoprogenitors and vascular cells will be probed for their roles in defining the hematopoietic stem cell (HSC) niche. The specific aims include (1) the development of microfluidic techniques to allow large-scale encapsulation of single cells in highly defined extracellular matrix mimics in order to determine how matrix cues regulate mesenchymal stem cell differentiation at the single cell level, (2) the creation of hybrid integrated circuit/microfluidic circuit systems to enable one to assemble picoliter drops containing individual cells and synthetic ECM into 3D assemblies with pre-defined structure and organization, and (3) determining whether appropriate in vitro assembly of HSCs and cells representative of the bone marrow HSC niche can yield functional hematopoietic tissues capable of recreating hematopoiesis in vitro. Success in this project will lead to the creation of a new set of tools that will enable formation of 3D tissues with precisely defined cell placement, and homotypic and heterotypic cell-cell interactions. These tools are likely to be broadly useful to the creation of new in vitro models of tissue development and drug screening, and in vivo tissue replacements from a variety of cell types. As stem cells are particularly sensitive to environmental cues, inappropriate cell-cell and cell-matrix interactions likely lead to the irreversible and undesirable alterations in stem cell differentiation fate found in culture. The systems developed in this project will allow us to investigate the specific role of vascular cells and osteoprogenitors/osteoblasts in maintaining the human HSC niche, which is a difficult question to address in vivo. It is crucial to better define and create models of the niche to understand normal hematopoiesis and pathologies involving blood cells, and to enable hematopoiesis on demand in various therapeutic venues. The key studies to date on this topic have relied on rodent models, and the relevance of many findings to human biology is currently unclear. (End of Reviewers' Comment)